X-ray imaging system and solid state detector therefor

ABSTRACT

The x-ray imaging system comprises an x-ray source for producing an x-ray beam and an x-ray detector. The x-ray detector comprises a solid state integrated circuit having a silicon substrate and a plurality of charge storage devices. A circuit is provided for placing a charge on the charge storage devices and the integrated circuit is disposed in an x-ray permeable material. The detector is positioned in an x-ray beam such that the charge is dissipated by secondary radiation produced by interaction between the x-ray beam and the silicon substrate of the integrated circuit.

CROSS REFERNCE TO RELATED APPLICATIONS

This is a continuation in part of U.S. application Ser. No. 807,650,filed Dec. 11, 1985.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to x-ray imaging systems and particularly to anx-ray imaging system which utilizes a solid state x-ray detector.

2. Discussion of Related Art

Presently, x-ray imaging systems are utilized in a variety ofapplications, both as medical diagnostic tools and for industrialquality control. The most common form of x-ray detection resides in theuse of silver halide film. However, the use of such film requires theperformance of several wet, control requiring chemical developing steps.In addition, this film is expensive, thus increasing the cost of x-rayimages produced in this manner.

It would be highly desirable, therefore, to produce an x-ray imagingsystem which does not require the use of silver halide film. Severaldetectors have been proposed for this purpose.

For example, U.S. Pat. No. 4,471,378 to Ng discloses a light andparticle image intensifier which includes a scintillator andphotocathode unit for converting incident image conveying light orcharged particles to photoelectrons and a charge coupled device fordetecting the photoelectrons and transmitting to data processing andvideo equipment information relating to the quantity or energy level aswell as the location of the electrons impinging on the sensing areas ofthe charge couple device.

U.S. Pat. No. 4,413,280 to Adlerstein et al discloses an x-ray imagingapparatus which includes a transducer for converting incidentx-radiation to a corresponding pattern of electrical charges. Thecharges generated by the transducer are accelerated onto an array ofcharge detecting or charge storing devices which store the charges inthe form of an electrical signal corresponding to the charge pattern.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a solid state imagingsystem and detector which are highly sensitive to x-radiation and canproduce highly accurate x-ray images.

Another object of the present invention is to provide an x-ray imagingsystem and detector which can be produced by conventional solid statefabrication technology.

A further object of the present invention is to provide a solid stateimaging detector which can be produced in such small sizes as to enableits use in very confined areas.

Another object of the present invention is to provide an x-ray imagingdetector which can be substituted directly for x-ray film used inconventional x-ray imaging systems.

Yet another object of the present invention is provide an x-ray detectorwhich is relatively inexpensive to fabricate so as to enable its use infixed locations for ease of periodic x-ray analysis of mechanicalstructures and the like.

In accordance with the above and other objects, the present invention isan x-ray imaging system comprising an x-ray source for producing anx-ray field, and an x-ray detector. The x-ray detector comprises a solidstate integrated circuit having a plurality of charge storage devicesand a circuit for placing a charge on the charge storage devices. Thecharge storage devices are disposed in an x-ray permeable material andthe detector is positioned in the x-ray field such that the charge isdissipated by secondary radiation produced by interaction of the x-rayfield in the silicon substrate of the solid state integrated circuit.

The charge storage devices may be divided into groups to form pixels.Each pixel comprises one or a plurality of charge storage devices andthe exposure times for discharging the charge storage devices in asingle pixel can be different from one another to provide a gray scale.

In accordance with other aspects of the invention, the integratedcircuit may be a dynamic random access memory.

Each charge storage device comprises a single cell of the integratedcircuit. The cells are spaced from each other such that dead spaceexists therebetween. Also, the cells are produced in banks of 32,000with about 1/4 mm dead space between banks. A plurality of detectors maybe stacked with the cells of the detectors staggered such that each cellof one detector is positioned behind the gap between cells of anotherdetector so as to eliminate all dead space.

The imaging system also includes processing circuitry for accessing thecells of a detector. The processing circuitry may include a system fornormalizing the outputs of all of the cells to compensate for variousinherent differences in radiation sensitivities of the various cells.

One of the most important aspects of the digital radiography techniqueemployed in the present invention compared to conventional systems usingsilver halide film is the ability to perform quantitative radiography.This is achieved practically through image digitization and makessubtraction of radiographic images an extremely useful enhancementtechnique.

The x-ray image detection system according to the present invention isbased on direct acquisition of digital information, utilizingsolid-state silicon and hybrid detectors. An x-ray image of an object isprojected directly onto the sensor without any intermediatex-ray-to-light conversion and signal magnification. Secondary electronsproduced by x-ray interactions with the silicon substrate are collectedand digitized using techniques similar to those employed for visiblelight detection.

One of the major concerns of direct x-ray sensing is designing asolid-state sensor that can withstand the radiation dose accumulationsufficiently to justify the cost of replacing the degraded detectors.The sensor must have good x-ray sensitivity compared to other systemswith typical x-ray spectra (30 kVp to 200 kVp), and should have acapability of sensing a continuous large format image. To solve thisproblem, the sensor used in the present invention is a convention DRAMdevice. The cost of producing such a device is orders of magnitude lessthan producing other types of sensors, such as CCD and CID arrays.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects of the present invention will become morereadily apparent as the invention is more clearly understood from thedetailed description to follow, reference being had to the accompanyingdrawings in which like reference numerals represent like partsthroughout, and in which:

FIG. 1 is a block diagram of the x-ray imaging system of the presentinvention;

FIG. 2 is a circuit diagram of an integrated circuit detector used inthe imaging system of Figure 1;

FIG. 3 is an enlarged schematic showing one charge storage capacitor ofthe circuit diagram of FIG. 2;

FIG. 4 is a cross section of a chip showing the structure depictedschematically in FIG. 3;

FIG. 5 is a view of a portion of the detector of the present inventionstacked over additional detectors to fill up the dead space betweencells;

FIG. 6 is an end elevational view of the stacked detectors of FIG. 5;

FIG. 7 is a diagrammatic representation showing the system of thepresent invention used in place of x-ray film;

FIG. 8 is a flow diagram depicting a method of normalizing the cells ofthe present invention;

FIG. 9 shows the orientation of the detector and the detector leads;

FIG. 10 is a graph showing pixel logic hold time as a function ofaccumulated radiation exposure based on data taken at 120 kVp filteredthrough 0.25 mm Al with 50% of total detecting pixels discharged beyondthe threshold point;

FIG. 11 is a graph showing pixel integration time as a function ofaccumulated exposure based on data taken at 120 kVp filtered through0.25 mm Al;

FIG. 12 shows a thin film detector embodiment of the present invention;

FIG. 13 is a cross section of one detector of the embodiment of FIG. 12;

FIG. 14 is a circuit diagram of the detector of FIG. 12; and

FIG. 15 is a cross section showing the interconnection between thesensing layer and the preprocessor portion of the detector of FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows the x-ray system 10 to comprise a high energy x-ray source12 and a detector 14 positioned to receive the radiation from source 12.Source 12 can be any standard high energy x-radiation source having anoutput in the range of 8 Kev or higher. Sources such as this are wellknown and manufactured by, for example, G.E. or Siemens. Alternatively,source 12 be an ultra small focal spot source such as manufactured byRidge or Magnaflux, also having an output in the range of 8 Kev orhigher. Any size focal spot source can be used. Currently, the smallestfocal spot available is one micron. Also, the source 12 and detector canbe placed as close to the object 0 to be x-rayed as desired due to theconfiguration of the detector 14, as will become readily apparent.

Detector 14 may be a dynamic random access memory such as the IS32OpticRAM sold by Micron Technology, Inc. of Boise, Idaho. This device isan integrated circuit DRAM having 65,536 elements and is used as a solidstate light sensitive detector. The Micron DRAM is specifically adaptedto sense light inasmuch as there is no opaque surface covering theintegrated circuit. However, any type of dynamic random access memorymay be used for detector 14 as long as the covering is transparent tox-radiation. In fact, as will become apparent, any type of dynamicmemory element may be used as detector 14. The memory element does nothave to be a random access memory, although the use of a random accessmemory facilitates preprocessing and image processing routines.

The IS32 OpticRAM image sensor is a solidstate device capable of sensingan image and translating it to digital computer-compatible signals. Thechip contains two arrays each of which contains 32,768 sensors arrangedas 128 rows by 256 columns of sensors (4,420 microns×876.8 microns).Each pixel, 6.4 microns on a side, consists of two elements, a MOScapacitor and a MOS switch. The fill factor is 50 percent. The sensor isa random access device and thus, pixels may be individually accessed.

The detector 14 operates by the projection of radiation penetrating theobject onto the 65,536 radiation-sensitive elements of the array-pair.Radiation striking a particular element will cause the capacitor, whichis initially charged to five volts, to discharge toward zero volts. Thecapacitor will discharge at a rate proportional to the intensity of theradiation field to which it is exposed.

To determine whether a particular element is black or white, one canread the appropriate row and column address associated with the physicallocation of the element. The sensor reads the voltage value of thecapacitor and performs a digital comparison between the voltage of thecapacitor and a fixed externally applied threshold voltage bias. A whitepixel indicates the capacitor is exposed to a radiation field sufficientto discharge the MOS capacitor below the threshold point, whereas ablack pixel has not received sufficient exposure.

The output of detector 14 is passed to preprocessor 16 which serves thefunction of normalizing the outputs of all of the cells of detector 14.That is, the sensitivity of the cells of detector 14 will inherentlyvary. A normalization value can be stored in preprocessor 16 so as tonormalize the output of each of the cells to ensure a field describingreading.

The normalized output of preprocessor 16 is passed to image processor 18which manipulates the data using conventional image processing programsas well as new image processing programs which will be made possible bythe present invention, such as "zoom" programs which are not currentlyin existence. This image can be displayed on a high resolution monitor20, can be stored on a laser disc recorder 22, can be printed using adry silver printer 24, or can be sent via satellite to remote imageprocessors (not shown). A menu driven program is displayed on a computermonitor 26 prompting appropriate instructions and data which can beentered into the image processor 18 using a keyboard 28.

FIG. 2 shows a schematic diagram of a portion of a typical DRAM used indetector 14. The circuit 30 comprises a plurality of cells 32, each ofwhich contains a memory capacitor 34 and an access transistor 36. Theindividual cells are accessed through left and right digit lines 38 and40, respectively, as well as word lines 42 and 44. A sense amplifier 46is provided in the form of a cross coupled MOSFET detector circuit. Thesense amplifier 46 has nodes A and B which are coupled to the left digitline 38 and to the right digit line 40, respectively. The cells 32 aredivided into a left array 50 and a right array 52. The left array 50 isaccessed by the left digit line 38 and the right array 52 is accessed bythe right digit line 40. The word lines 42 access the individual cellsof array 50 and the word lines 44 access cells of the array 52.

A pair of equilibrate transistors 56 and 58 couple the digit linestogether to allow equalization of the digit lines at the end of arefresh cycle and during the recharge state of the next cycle.

The common drains of the cross coupled sense amplifier transistors atnode C are connected through an isolation transistor 60 to a pad 62 onthe periphery of the integrated circuit chip. The pad 62 is bonded toone of the leads of the circuit chip package.

A pair of pull up circuits 66, 68 are coupled, respectively, to thenodes A and B. The pull up circuits 66, 68 are voltage divider circuitsoperable to control the voltage level of the digit lines 38 and 40.

FIG. 3 shows one cell of the circuit 30. For convenience, the cell isshown to be one of the array 52 but it could be any of the cells. Asshown, the capacitor 34 has two plates 70 and 72 between which a chargeis stored. Initially, the capacitor is charged by applying a highpotential on word line 44 and a high potential on right digit line 40.This corresponds to a "1" state of the cell. In the presence of incidentx-radiation, the charge on capacitor 34 is dissipated as will bediscussed below.

With reference to FIG. 4, the portion of the integrated circuitcontaining the cell shown in FIG. 3 is set forth in cross section. Thecircuit comprises a p-type silicon substrate 80 onto which an n+ region82 has been added. A silicon dioxide layer 84 is deposited over thesubstrate and n+region 82 to form an insulating layer. The lead 40 isconnected to the n+ region to form the drain of transistor 34. A metalplate 86 is formed on the oxide layer 84 to form an insulated gate oftransistor 36. The capacitor 34 is formed by a metal plate 72 and theinterface 70 between p-type substrate 80 and oxide layer 84, which formsthe other capacitor plate.

When the cell of FIGS. 3 and 4 is set to the "1" state, charge is builtup on the interface 70 to charge the capacitor 34. The gate voltage isthen lowered so as to discontinue communication between the drainvoltage at line 40 and the capacitor 34. This charge is dissipated dueto the absorbtion of x-ray photons in the substrate 80. In FIG. 4, thedirection of incident x-radiation is shown by the arrow 88.

The x-radiation can produce free electrons in the substrate 80 either byphotoelectric effect, Compton scattering, or pair production. However,because of the high energy of the source used in the present system, thenumber of electrons produced through photoelectric effect is negligible.The x-ray energy is in the range where compton scattering and pairproduction have the highest probabilities of producing free electrons.

It is noted that interconnections between components of a cell andbetween cells are provided on the oxide layer of the semi-conductor.This is indicated in FIG. 4 by showing the leads 40, 44 a 45 extendingout of the oxide layer. Such leads represent the interconnectionsproduced by the metalization layer of an integrated circuit.

Irradiation of the cell 30 from either side results in virtually all ofthe x-radiation being received in the substrate 80 so that the totalelectron production for any given energy level of radiation is achieved.The free electrons produced by interaction between the substrate and thex-radiation decrease the charge at junction 70 and thus decrease thecharge on capacitor 34.

In conventional x-ray systems, compton scattering and the photoelectriceffect are the relevant interactions causing the appearance of freeelectrons. Rayleigh scattering, a type of coherent scattering may alsobe responsible for the production of some free electrons. The relativeoccurrences of the different reactions depends on the energy of thex-ray. As discussed above, the present invention uses a high energysource. Rayleigh scattering and photoelectric effect are low energyinteractions so that the number of free electrons produced by the theseeffects in the present invention is negligible. There is littledirection sensitivity in any of the interactions relating to theproduction of free electrons except in the case of Rayleigh scattering,which is predominantly forward but also yields the least free electrons.

Referring again to FIG. 2, it will be understood that the circuit 30 isof the type which employs a dynamic/active/restore sense amplifier ofthe type described in U.S. Pat. No. 4,397,002, issued Aug. 2, 1983 toWilson et al, and U.S. Pat. No. 4,291,392, issued Sept. 22, 1981 toProebsting.

In operation of circuit 30, during one cycle, a given word line 42, 44in FIG. 2 is brought to a logic one level to enable the addressed accesstransistor 36. The respective cell capacitor 34 is discharged into theappropriate digit line (e.g. digit line 40 for a capacitor of array 32)changing its value above the equalized value. Then, a latch signal fromthe pad 62 becomes a logic low state to enable operation of the crosscoupled transistors and the sense amplifier 46 during absence of theequilibrate signal. The sense amplifier 46 responds to the latch signalby reducing the opposite digit line (in this case digit line 38) to aground potential. The digit lines are connected by input/outputcircuitry (not shown), which provides a digital signal representing thecontent of the selected memory capacitor 36. The pull up circuits causethe right digit line to be pulled up to the level of the supply voltage.At approximately this time, the storage capacitor 36 which has beenconnected to the bit line has been restored to its original logic onestate. The word line is then returned to ground to isolate the charge onthe respective memory cell. The digit lines are then permitted to go lowand the equilibration signal becomes a logic high to render theequilibrate transistors 56, 58 conductive to allow the digit lines to beconnected for equalization. This permits the charge on the digit lines38, 40 to be shared such that the digit lines equilibrate to a voltageapproximately half way between the supply voltage and ground. A newcycle is thereupon ready to commence.

A factor affecting the performance of the detector is the length of thetime which the MOS capacitors are exposed to the radiation field. Thisperiod of time is measured from the initial exposure of an element untilthe time the particular element is read or refreshed. Accessing anypixel in a row causes the entire row to be refreshed. This sets all therow cells that have not leaked below threshold to five volts and setsall cells in the row that have leaked below threshold to zero volts.Optimal imaging conditions exist when the absorbed dose rate is muchgreater than the discharge rate created by dark current in the capacitorcircuit.

The latch signal is placed on pad 62 such that during the equilibratesignal, a voltage potential may be applied to the digit lines when theyare connected to effect equilibrium. As will become apparent, thevoltage which is applied to the pad 62 allows adjustment to thesensitivity of the image sensor. In particular, the digit line potentialacts as a threshold to determine if a particular memory cell 36 is at ahigh or low voltage level. By raising the potential, cells may leak lessbefore they are considered to be decayed from a logic one to a logiczero value. Accordingly, the sensitivity of the cells can be adjusted byadjusting this potential.

Another way of adjusting the sensitivity of the cells is by etching indifferent thicknesses of oxide material in the capacitor 34 shown inFIG. 4. In other words, the thickness of the oxide between interface 70and metal plate 72 determines the sensitivity of the capacitor tox-radiation. The thicker this layer, the more sensitive the capacitor isto radiation discharge. Accordingly, to make a more sensitive detector,the layer should be made thicker and to make a less sensitive detector,the layer should be made thinner. There are, however, limitations tothis technique because the impedance of the capacitor must lie in acertain range in order for the circuit to function properly (varying thethickness changes the impedance).

A further technique for adjusting sensitivity is to adjust the exposuretime of the detector cells. Using conventional x-ray source controllingequipment, the response of the cells can be adjusted either by usinghigher energy x-rays or by increasing the intensity of the x-rays.Clearly, in either case, the cells will react more quickly.

The OpticRAM has a broad spectral sensitivity range. In theUV-visible-IR portion of the spectrum (300-1,200 nanometers) thesensitivity is fairly uniform requiring a fluence of about 2 microjoulesper square centimeter to discharge the sensor to threshold (typically2.5 volts). In the x-ray region of the spectrum the fluence required toreach threshold is about 0.2 microjoules per square centimeter (at 120kVp). The detector responds almost linearly with x-ray energy spectrafrom 20 kVp to 120 kVp. The response of the sensor below 15 kVp is lowtotally due to the strong absorption of the filter employed. In fact,most solid-state planar devices have optimum sensitivity in the photonenergy range from 1keV to 30 keV.

The inherent x-ray sensitivity of a silicon sensor is a function of thedevice structure, process parameters and cell configuration. The usualIS32 OpticRAM is not in the optimum condition in any of those threeaspects. The dielectric layer of the device was modified to examine itseffect on sensitivity. Test results show that increasing the gate oxidelayer thickness from 300 Angstroms to 600 Angstroms increases thedetector sensitivity by an order of magnitude. This is interestingsince, by doubling gate oxide thickness, one not only doubles the activevolume for x-ray interaction in the oxide region (assuming depletionregion remained unaffected) but also reduces the total number of eventsof x-ray interaction necessary for the MOS capacitor to discharge belowthe threshold point. The expected improvement is about four fold, ratherthan the 10 fold observed int he measurement. Experiments on the opticalsensitivity show only a three fold improvement with the same change. Itis not clear what constitutes the dramatic x-ray sensitivity improvement(two-and-one-half times greater than expected). There is an indicationthat this method may have a limit. In an experiment conducted on anIS6410 OpticRAM, and increase in oxide layer from 600 Angstroms to 800L- Angstroms improved detector sensitivity by only 25 percent.

One of the device structural parameters which has been changed isremoval of a silicon nitride layer normally deposited on the gate oxidelayer of the capacitor. The nitride layer is common in DRAMs to reducebit soft-errors, but also appears to reduce x-ray sensitivity by abouttwo orders of magnitude. A possible explanation for this observation isthat the nitride layer has a greater electron trapping density than theoxide layer. As a result, a significant portion of signal carrierscreated by a radiation field may be trapped and thus not collected bythe pickup circuit.

The x-ray sensitivity of the modified IS32 OpticRAM is in a usefulrange. Approximately 50 milliRoentgen is sufficient to trigger the IS32to acquire a binary image exposed to an x-ray energy spectrum of 30 to120 kVp with 0.25 millimeter aluminum filtration.

Experiments conducted thus far have focused on sensitivity improvementsachieved by adjusting parameters that can be easily modified. Already,the sensitivity of the IS32 has been improved to a degree close to thatof bare film. It is expected that at least another order-of-magnitudeimprovement is possible without requiring a major change in fabricationprocess and cell configuration.

Sensitivity variation from pixel to pixel is another major concern.Typically, there is a five percent inherent quantum statistical noisepresent in each pixel and 20 percent bitwise nonrandom sensitivityvariation created during fabrication. Though the 20 percent sensitivityvariation is nonrandom and can be normalized either by software orhardware, its presence is not desirable because of the additional noiseintroduced through normalization procedures and the extra time requiredfor image normalization. Several fabrication techniques exist forcreating a chip with reasonable sensitivity uniformity (e.g. fivepercent) including uniform sense line capacitance and a reduction inspatial uncertainty in impurity doping and the oxide laying process.

The cells of the detector are binary in nature. In many applications, itis useful to have a gray scale. This can be accomplished using any ofthe three techniques discussed above for varying the sensitivity of thecells.

If the digit line potential threshold is varied to vary the sensitivity,a plurality of cells may be grouped to form a pixel. For example, 8cells per pixel could provide a gray scale having 7 levels, although inpractice, 80 cells per pixel (79 gray scales) may be used to provideredundancy at each level of the gray scale. In FIG. 2, if it is assumedthat array 32 has 8 cells, ideally, this array could act as a singlepixel. During the cyclic operation of the circuit 30, as discussedabove, a different digit line potential is applied to digit line 40 whenreading each of the memory capacitors 34 of array 32. In this manner,the soak time required for each of the cells 32 to discharge to a lowlevel would be different, thereby providing a gray scale.

The preprocessing circuit 16 of FIG. 1 is programmed to provide thenecessary variations in threshold potential, as would be apparent to oneof ordinary skill in the art.

Likewise, if each of the cells of array 32 forms a single pixel, eachmemory capacitor 34 could be produced with a different thickness ofoxide layer. In this manner, the capacitors would have inherentdiffering sensitivities, thus producing a gray scale. Clearly, thenumber of cells in a pixel can be varied to produce a gray scale havingas many levels as desired.

A third means of providing gray scale is to take several x-ray pictureswith different soak times and to add the results. For example, with anobject having a varying thickness, a plurality of images can be obtainedusing differing soak times. The lower soak times are used to depict thethinner portions of the object. By adding all of the images together, amultilevel gray scale image is obtained depicting the variousthicknesses of the object.

The detector 14 can be produced in any desirable size. Conventionally,the largest silicon chips produced are 6" diameter circular wafers. Thewafers are cut into individual integrated circuits and packaged as, forexample, dual in-line packs. The detector 14 can be processed byconventional integrated circuit processing techniques and can bepackaged in any conventional configuration such as the dual in-linepack.

FIG. 5 shows a portion of a detector 14 having cells 100. The cells areeach approximately 8 microns square and the cells of any row are spaced9 microns from center to center. Thus, these cells are separated by onlyone micron. Vertically, as shown in FIG. 5, the cells are spaced 25microns from center to center. Also, the cells are arranged in banks of32,000 with approximately 1/4 dead space between the banks. The deadspace between cell banks is required to accommodate the trunk lines tothe cells. The arrangement of cells in one integrated circuit issufficiently dense to provide a very high resolution x-ray image formost applications. However, if an even higher resolution is required,since each chip is relatively transparent to radiation, a plurality ofchips can be stacked and offset relative to one another to fill the gapsbetween cells and/or between the banks of cells. For example, as shownin FIGS. 5 and 6, three detectors 14 are stacked and offset so that thecells of the lower two detectors are positioned between the cells of theupper detector. In FIG. 5, the cells of the lower detectors areindicated in phantom as cells 100' and 100". Accordingly, it can be seenthat a single detector can be built using three integrated circuits inwhich virtually all of the available area is filled with cells.

As an alternative to stacking detectors, several sequential images canbe taken with either the source 12 (FIG. 1) or the detector 14 beingmoved between each x-ray exposure. This motion is produced byoscillating supports 102 or 104 shown in FIG. 1.

The integrated circuit itself is about 1/2 mm thick. The entire detectorcan be made about 1/2 inch thick, including all necessary connectionsetc. Accordingly, a detector 14 can be substituted directly for x-rayfilm in almost all existing x-ray devices. As shown in FIG. 7, thedetector 14 can fit directly into an x-ray film receptacle 110 withleads 112 and 114 being connected to the processing section 116 whichcontains preprocessor 16, image processor 18, etc. Processing section116 may be a computer with the preprocessor and image processor beingsoftware components.

Another advantage of the fact that the detector 14 can be produced insmall sizes is that relatively small detectors can be permanentlylocated in virtually inaccessible areas of structures to be x-rayed. Forexample, structural elements in aircraft are required to be examinedperiodically using x-ray techniques. This normally requires disassemblyof major structural components. However, with the present invention, anx-ray detector can be permanently mounted in place and the detectorleads can be made accessible through a plug or the like. In this manner,x-ray analysis of structural components can be carried out quickly andeasily. Also, since the detector 14 is made using conventionalintegrated circuit techniques, its cost is relatively low thus alsocontributing to the feasibility of utilizing a plurality of small x-raydetectors 14 on a component requiring x-ray analysis.

Furthermore, the small size of the x-ray detector 14 enables it to beplaced in body cavities such as the ear or the like to facilitate clearclinical diagnostic x-ray images.

The fact that each cell of detector 14 is small in size enables the useof an x-ray source 12 having the smallest focal spot available. Also,since the cells are relatively closely packed, the present invention canbe used in zoom radiography without requiring any change in the distancebetween the source and the detector. Normally, the size of an x-rayimage can be increased by increasing the distance between the object andthe detector. However, in the present invention, due to the large numberof cells in the detector, the image can be electronically enlargedwithout significant loss of resolution by merely displaying the image ona larger fraction of the screen. Further, as discussed above, ifadditional resolution is required, detectors 14 can be stacked as shownin FIGS. 5 and 6.

Due to the small size of the cells of the invention, greater resolutioncan be achieved with little or no cross talk between the cells. That is,the length of travel of the most energetic secondary radiation producedby interactions between the x-ray beam and the silicon substrate 80(FIG. 4) is approximately 10 microns. Since this is also the separationdistance between cells, there is very little probability that radiationimpinging upon one cell will result in noise being produced in anadjacent cell.

Furthermore, the present invention is suited for microradiography sincethe cells are small and closely packed. The resolution attainable usingthe invention is sufficiently high to permit an accurate representationof objects having an area on the order of 1,000 square microns.

Also, because of the digital nature of the present invention and itsinherent compactness, it is possible to configure an x-ray imagingsystem in a unique way that permits the user to view the internalstructure of objects where it is not possible or practical to place theimaging media behind the object. As shown in FIG. 9, the x-ray source 12is placed directly behind the detector 14 which can be placed directlyon or in front of the object 0 to be viewed. In this configuration, thex-ray field produced by the source penetrates through the detector andimpinges on the object. The image of the object is created in thedetector by back scattered radiation from the object. The image producedin the detector 14 is that of the object 0 produced by back scatteringsuperimposed on an image of the source 12 produced by the x-ray fieldpassing through the detector 12 initially. The image of the source 12can be removed by subtracting it from the total image. That is, an imageof the source alone is produced by irradiation of the detector 14 withno object present and digitally subtracting this image from that of thecombined object and source image.

It should be noted that it is preferable to orient detector 14 in FIG. 9such that the connection leads extend out of the side facing the source.In this way, the detector can be positioned closer to the object 0 andthe image of the leads, etc. will be present when the image of thesource alone is produced and thus will not appear in the final imageafter digital subtraction.

Furthermore, by varying the energy of the x-ray field with multipleexposures, back scatter tomography is made possible, wherein a 3-D imageof the object can be constructed.

The measured inherent sensitivity of cells of a detector varies by about20%. Furthermore, it has been discovered that the sensitivity of thedetector 14 is varied by prolonged exposure to x-radiation. That is, ascells of the detector are exposed, they become more sensitized toradiation in the future. Thus, detectors which receive less radiationdose because they are shadow shielded by an object are less sensitizedthan those exposed to direct or less attenuated radiation. Therefore,the relative sensitivity of each detector varies according to its ownhistory of exposures. X-ray applications with typically useful imagegray scales do not allow such large sensitivity variations and thusrequire sensitivity normalization. Such normalization can be carried outusing the following technique. For the following discussion it isassumed that the detector comprises one half of a basic IS32 detectorconfiguration, i.e. a 128×256 cell array contained in a spatial area876.8×4420 microns. The cells are arranged in 128 rows and 256 columns.Presuming that the cells are illuminated by a source which generates auniform intensity field at the sensor, a distribution of "exposuretimes", t, will be obtained to register a cell discharge. The individualcells are denoted by t(m,n) where m is 1,2,3, . . . ,M and n is 1,2,3, .. . ,N. The letter m signifies the row index and the letter n signifiesthe column index. In the present example, M is 128 and N is 256.Denoting the average value of this array of 32,768 values by t*, notethat the 20% variability discussed above means that the difference oft(m,n) and t* to be as large as 0.2 t*. The cell array normalizationfactors can be defined by f(m,n) equals the ratio of t* to t(m,n). Thuswe can generate the array f(m,n) where m is 1,2, . . . ,M and n is 1,2,. . . ,N. In measuring the set of values t(m,n) we must set a timeincrement used to step through the cells. Certainly this incrementshould be less than 0.2 t* or the measurement will be of little value.

Where gray scale is produced by sequential exposures of the object withdifferent exposure times, the gray scale will be specified by a set ofcontiguous exposure time intervals. Denote the increasing bounds ofthese intervals by t(j) where j is 0,1,2, . . . ,J. This defines aJ-gray scale image mode (typically J is 8, 16, 32 or 64) bounded by alow exposure time of t(0) and a high exposure time of t(J). The j-thshade of gray is bounded by exposure times of t(j-1) and t(j). The imageintensity measured by the radiation field at sensor locations variesinversely with j.

In a particular image obtained with the J-gray scale just described, anarray of values t(m,n) will be obtained where m is 1,2, . . . ,M* and nis 1,2, . . . ,N*. The values of M* and N* are respectively equal to orless than M and N depending on either the fraction of cells used, orcell spatial averaging procedures. This array of cell exposure timescontains the image information. Since image exposure times are only atthe specified values t(0), t(1), . . . ,t(J) the image values of t(m,n)will fall in one of the discrete J gray scale intervals and no furtherdetailed knowledge can be presumed. Each cell response to the image mustbe corrected by the sensitivity normalization f(m,n). There are avariety of ways that this correction can be applied.

One method of carrying out this correction is shown by the program ofFIG. 8. This program is run to read each cell at each exposure. In otherwords, the first exposure time t(0) may be 1.1 sec, the second exposuretime t(1) may be 1.2 sec, etc. After each exposure, all of the cellst(m,n) are read and the information output from each cell is stored in amatrix.

In FIG. 8, the program is started at step 120 and the variables areinitialized at step 122. This includes setting m and n equal to one. Atstep 124, the value of the first cell t(m,n) is read and thenormalization factor for that cell is looked up at step 126. At step128, the gray scale interval j is determined. For example, if the cellsare being read after the first exposure and the exposure time is 1.1sec, the gray scale interval j would be 1.1-1.2 sec. If the secondexposure time is 1.2 sec, the second gray scale interval would be 1.1-12sec, etc.

At step 130, an exposure time t*(j) characteristic of the j-th intervalfor each j is identified. For example, t*(j) could be the simpleaverage: one-half of the sum of t(j) and t(j-1). In other words, if theinterval 1.1-12 sec is the current interval j, it is necessary to assigna single value t*(j) to the exposure time. This value may be, e.g., 1.15sec. Then, at step 132, for each image spatial location (m,n) which hasan image value t(m,n) in gray scale interval j, calculate f(m,n)t*(j)which yields the corrected value t*(m,n). This value: (1) falls ininterval j and then the assignment of gray shade j for (m,n) ismaintained; (2) falls in some other interval j' and then the assignmentof corrected gray shade j' for (m,n) is accomplished; (3) is less thant(0) and then (m,n) is assigned "white"; or (4) is greater than t(J) andthen (m,n) is assigned "black". The determination of the gray scaleinterval for t*(m,n) is made at step 134 and the actual assignment ofthe gray scale is made at step 136. Clearly the choice of t*(j) and thevalidity of the procedure is less uncertain when the intervals t(j-1) tot(j) are "small". Small is relative to expected cell sensitivityvariation over the image in question. At step 138, a determination ismade as to whether all cells have been read. If the answer is "no" thevalues of m and n are incremented at step 140 according to apredetermined schedule and the value of the next cell is read. After allthe cells are read, the program ends at step 142.

Silicon planar devices including the IS32 OpticRAM are sensitive toradiation damage. Two of the major effects of radiation damage to MOSsensors are: (1) positive space charge formation in the oxide layer, and(2) surface fast-state generation at the siliconoxide interface.

It has been shown that the charge buildup causes shifts in the operatingpoint of the MOS switch, catastrophic increase in the reverse current ofthe p-n junction and variations in their breakdown voltage. The increasein fast surface state density is responsible for the lowering of thetransconductance of MOS switches, and, in combination with the spacecharge buildup, for the reverse current increase in the MOS capacitor.

A fresh IS32 OpticRAM with no prior history of radiation exposurenormally possesses a 40 to 50 second logic holdtime (i.e., time requiredfor a precharged pixel to deplete its logic state from high to low in anunilluminated condition) at room temperature. The logic holdtimedecreases with accumulated radiation exposure due to increased darkcurrent. FIG. 10 gives a typical representation of the logic holdtime asa function of the total exposure of the IS32. As can be seen, an averagepixel can maintain a high logic state for only one second after two kRaccumulated exposure. The chip functions irregularly as the logicholdtime approaches zero and eventually fails catastrophically. Thedegradation of chip performance as a direct x-ray sensor can best beobserved in FIG. 11 which is compiled from experiments performed tostudy the effect of leakage current on sensor interrogation time at roomtemperature. As shown in the figure, dark current becomes significant attwo kR accumulated exposure, and at eight kR exposure it contributesmore than 50 percent of the charge released within the capacitor for thesame exposure conditions. To reduce the dark current contribution, agreater radiation intensity is required.

The radiation induced damage results in detectors with varying logichold times (or DC leakage rates) depending on the cumulative amount ofdose absorbed by each detector. Thus, it is possible to create latentimages on the sensor array by taking repeated images of the same object.Detectors in the shadow of the object will be less affected by radiationthan those exposed directly to the illuminating field. After many imageshave been acquired with a detector array, a fixed pattern of detectorswith different DC leakage rates is created. Fortunately, the DC leakagerate differences between detectors may be normalized in the same way the20 percent nonrandom bitwise sensitivity variation created duringmanufacture is normalized. Under such circumstances, normalizationmatrixes are needed for raw image processing; one to smooth out thesensitivity variation due to the built-in uncertainty originated fromthe fabrication process and the other one for those variations due tothe performance degradation from radiation damage. Practically, onemerely obtains a new single normalization matrix periodically during theuseful lifetime of the chip. This periodically-updated normalizationmatrix is then employed for the next exposure interval. Presumably, thechip is not functional as an image sensor if either of the followingthresholds is reached:

(1) the noise level from the normalization of DC leakage rate variationsand nonrandom noise pattern is significant compared to the signalamplitude (say 10 percent), or

(2) the DC leakage level is so large that the signal content in the celldoes not provide sufficient quantum statistical validity.

The effective life of the detector, which is a function of absorbeddose, chip operating temperature and other parameters, is on the orderof 10 kR. Cooling and periodic UV annealing should significantly prolongthe useful lifetime of the chip.

The IS6410 OpticRAM, also produced by Micron, has a spatialconfiguration which makes it useful for testing the performance of thechip as a direct x-ray imager. It is composed of two rows of IS32OpticRAM array-pairs, five in each row, in a single die (FIG. 5). Onlythe top or bottom eight discrete array-pairs are utilized (for 8-bitprocessing). It has 524,288 pixels in an area of 0.8 square centimeter.The active area is actually 0.67 square cm with a "windowpane"conducting area occupying 0.13 square centimeter.

The IS6410 was interfaced with an image processor in the form of an IBMPC/AT. Since this detector is in fact a DRAM, the interface consists ofan extended memory controller board and a driver circuit. The chip isdesigned to produce only one binary image per exposure because the cellis read out destructively. The present approach to build up an imagegray scale uses integration time. This requires a separate exposure foreach gray level, each of which is longer than the previous exposure.Therefore, to obtain a picture of N gray levels, a total of N-1exposures is required.

The spatial and contrast resolution of the detector is determined by anumber of factors. The spatial resolution is determined by the larger ofeither the pitch of sensing pixels (in this case less than 10 microns)or the range of the secondary particles generated from x-ray interaction(Compton electrons). For typical x-ray energies (between 30 keV and 150keV) the range of Compton electrons within silicon is also on the orderof 10 microns. Thus, the spatial resolution of this device is about 10microns or about 40 line-pairs per millimeter. The contrast resolutionis ultimately limited by quantum noise. Since we can arbitrarily set theintegration time, the contrast resolution is also arbitrary. As apractical matter, a one percent signal (Contrast) variation is theminimum detectable variation.

The fact that the device is also a memory device leads to new conceptsin electronic imaging. For example, the x-ray source could be controlledby the detector, or various parts of the sensor could vary theirintegration time. The image processor could interrogate the sensors todetermine if they have received adequate exposure. Through a feedbackmechanism, the exposure could be controlled so that there were nounderexposed or overexposed sensors. This concept makes possibleintelligent image sensors (retinas) or machine vision, opening newhorizons for information management.

The IS6410 OpticRAM is found magnitude less sensitive to x-rays than itscounterpart IS32 OpticRAM. Tests conducted to compare logic holdtime ofboth chips as a function of total radiation exposure indicate bothsuffer the same rate of performance degradation from radiation. Thisimplies that pixels of both chips have the same x-ray conversionefficiency but different charge coupling efficiency. One plausibleexplanation of why these two sensors, which basically share the samecell layout, may exhibit different x-ray sensitivity is that, in orderto stagger 10 IS32 array-pairs on a single die and package the die in astandard carrier, changes of chip circuit layout are required. As aresult of the changes, the efficiency and the ability to read theinformation stored on the MOS capacitor can be hampered. A smallerportion of radiation-induced carriers passes through the sense line andis successfully collected by the pickup circuit. It is expected that inthe next generation sensor where wafer scale integration is required,the problem of reading information stored in pixels will besignificantly resolved.

There are many aspects of the device, such as the circuit architectureand the control logic, that require changes in its present arrangementin order to optimize its function as an x-ray imaging device. Threemajor areas requiring improvement are: (1) detector sensitivity tox-rays, (2) complete gray scale readout from a single exposure, and (3)scale-up to conventional image format size.

Low x-ray sensitivity is mainly caused by the effect of capacitanceloading and the small active volume of the capacitor. To alleviate theproblem of low x-ray sensitivity, several steps can be taken, includingbut not limited to increasing capacitor (detector) volume and area,optimizing the ratio of the sense line impedance and detector impedance,and increasing the sensitivity of the comparator circuit. Cellcapacitance can be increased by depositing a MOS capacitor with a largerarea and/or laying down a high dielectric medium (e.g. SiN_(x)) inaddition to the silicon oxide. The cell capacitance alters theefficiency of charge collection induced by the radiation field, and alsoalters the efficiency and ability to read information stored on thecapacitor. The absolute value of the capacitance of the sensing elementplays a role in determining the sensitivity of the sensor to theradiation field. The ratio of the cell capacitance to the sense linecapacitance affects the cell's efficiency, by affecting the user'sability to read the information or to measure the charge on thecapacitor.

Several fabrication techniques exist for creating low capacitance senselines including double diffusion and metal deposition. The primary taskin this step is to combine the various fabrication techniques to producea chip with optimum sensitivity and uniformity.

The IS6410 OpticRAM produces only binary image information per exposurebecause the cell is read out destructively. This requires a separateexposure for each gray level, each of which is longer than the previousexposure. Therefore, to obtain a picture of 256 gray shades, a total of255 exposures is required. This is an undesirable situation and producesa very slow and radiation intensive imaging device.

Several cell configurations are available to correct the currentlimitations of binary image sensing. These include: (1) an analog DRAM,(2) a cell with internal gain, and (3) a nondestructive readout (NDRO)cell with an internal comparator.

The analog DRAM is basically an X-Y addressable MOS imager, such as theMOS imager manufactured by Hitachi because the Hitachi device has ananalog readout, although the Hitachi device is not random access. TheHitachi MOS imager requires no major modification to make it a directx-ray imager, but it has a low charge coupling efficiency. Cells withinternal gain, such as the static induction transistor and the chargemodulation detector are attractive, but are still in the research anddevelopment stage. The NDRO cell will have no problem with chargereadout because of the short distance signal carriers are required totravel. However, it is potentially a very slow imaging device due to thenumber of times a pixel must be accessed to build a complete picture.

Selection of the best cell configuration is a complicated task whichinvolves consideration of structural parameters and chip fabricationprocessing constraints. At this writing, it is not clear which is thebest configuration of each concept or which concept is superior.

Direct x-ray imaging techniques, though producing large magnificationand high spatial resolution, suffer the drawback of a small image formatsize. Currently, the largest existing solid-state detector size fordirect x-ray detection is approximately one-half inch in diameter (e.g.,direct x-ray vidicon tubes manufactured by Hamamatsu Photonic Systems inWaltham, Massachusetts, or Teltron, Inc., in Douglasville, Pa.). Toexpand the field of view of a solid-state silicon detector to a levelsignificantly greater than the current capability, several techniquesincluding wafer scale integration are under investigation. One possibleapproach is to bond out entire wafers containing thousands of IS32cells. Four major problems are foreseen: (1) dead silicon space betweensensor chips, (2) effect of capacitance loading on sense lines, (3)fixed pattern noise, and, (4) replacement cost increases with size ofimage field-of-view.

The dead silicon area will cause a "windowpane" effect on images. Toreduce the windowpane effect one can, during the image taking process,either perform multiple exposures at various view angles until the wholepicture is covered or stack multilayer thin wafers, one on top ofanother, filling up the dead space in other wafers. Only two or threewafers are required to fill up the blind space depending on the chiparrangement on the wafer.

Effects of capacitance loading on sense line and fixed pattern noisewill be significant if image information has to be down loaded directlyfrom a pixel to the edge of each wafer. Signal conversion to digitalform n the chip level is an attractive approach to mitigate thesensitivity problem caused by the capacitance loading of the sense lineand the resulting fixed pattern noise. The main issue here is to expandthe image format to a reasonable size without sacrificing uniformity ofpixel information readout over this entire imaging surface.

The previous descriptions of detector 14 relate to monolithic devices.However, advances have been made in thin film technology in whichsemiconductor, metal and insulator layers are deposited sequentially toform a device. This type of process is particularly well suited to thefabrication of x-ray detectors incorporating MOS capacitors, asdiscussed above, in addition to an absorber layer having an atomicnumber Z which is higher than silicon so that the mass-energy transfercoefficient of the detector is increased. This type of technology alsoenables the absorber to be placed close to the MOS capacitors so thatfree electrons produced in the absorber can directly interact with thecapacitors. In other words, at the x-ray energy level of interest, freeelectrons are produced in the absorber primarily due to Compton eventswith high quantum efficiency. These free electrons affect the charge onthe MOS capacitors. Since the quantum efficiency of the absorber ishigher than that of silicon alone, the change in charge on the capacitoris greater and the sensitivity of the device is improved.

Many silicon on insulator (SOI) devices have been proposed and thefabrication techniques involved in their production are well known andwill not be discussed in detail here. The following discussion willfocus on an embodiment of the present invention in the form of a solidstate x-ray image acquisition device comprising a matrix of twodimensional silicon photodiode arrays coupled directly through indiumbumps to a matrix of silicon based preprocessors.

FIG. 12 shows the basic layout of the device which includes aninsulating substrate 200 to provide mechanical support, a plurality ofpreprocessor chips 202, and a detector layer 204 which includes anothermechanical support 206. The detector layer 204 contains a plurality ofMOS sensing elements and circuitry for randomly accessing the sensingelements. The outputs of the sensing elements are connected to thepreprocessors 202 through indium bumps 208 and the outputs of thepreprocessors 202 are taken through a conductive tab film 210.

FIG. 13 shows the composition of the detector layer 204 which containsthe support 206. Support 206 may be an amorphous silicon substrate as isconventional in thin film technology. An x-ray absorber layer 212 isformed on the support 206 by any convenient process such as vacuumdeposition. The x-ray absorber layer 212 should be formed of a materialhaving a high atomic number but should not alter the thermal propertiesof the device. Tungsten or tungsten carbide has the properties requiredfor a good absorber. That is, these materials have a higher atomicnumber than silicon and but have a thermal expansion coefficient whichwill not affect the integrity of the device. However, any other materialhaving the required characteristics may also be used.

A plurality of individual sensing devices 215 are formed on the absorberlayer. The sensing devices are formed in blocks arranged in a matrixwherein each block consists of 512×512 sensing elements and all thenecessary conductors and connection pads. Each sensing device occupiesan area about 0.1 mm square and includes a P-type polycrystallinesilicon layer 214 formed on the absorber layer 212 and a silicon dioxidelayer 213 on the polycrystalline silicon layer 214. Each device includesa sensing area 216, a y MOS switch 218 and an x MOS switch 220. Eachsensing device is the same as one of the MOS sensors discussed aboverelative the monolithic structures. The sensing area 216 is an MOScapacitor whose charge is altered by the free electrons produced inresponse to the Compton events in the absorber layer 212. Each sensingarea 216 capacitor should be relatively large in order to improvesensitivity. Preferably, each capacitor should be about 50 microns on aside, although a smaller size capacitor may also work as well. Eachsensing device is essentially one cell of a DRAM and the charges on thecapacitors can be individually read out by accessing the cells throughthe x and y switches using word lines and digit lines which control thelevel on the y MOS switch line 219 and the x MOS switch line 220. Thesignal on the capacitor is read out on line 224.

FIG. 14 shows the layout of a portion of one block of sensing devices215. With a basic cell size of 0.1 mm square, the sensing devices of thepresent invention can be used to form a high resolution x-ray detector.The connections to the sensing devices are made through indium bumps 208to the preprocessors below.

With the above described configuration, a large size matrix such as 14inches by 17 inches can be built by adding more blocks. The final sizewill be limited mainly by the fabrication process. With each sensingdevice occupying an area of about 0.1 square mm, it will take as many as14.336 million sensing elements or 7×8 blocks to have a 14 inch×17 inchsquare x-ray imager, As each sensor block is coupled to a preprocessor,it requires 56 preprocessors arranged in 7×8 format to accommodate allof the sensing elements.

FIG. 15 shows the structure for connecting the switching and signallines of the sensing elements to the preprocessors 202, which are formedon a layer below the sensing elements. As seen in FIG. 14, an indiumbump extends from the preprocessor 202 to the associated sensing elementformed in layer 204. One indium bump is provided for each suchconnection. The indium bump shown in the figure is for connecting thesensor output line 224 to the input line 232 of the preprocessor. Aswill be understood, a similar indium bump is provided for eachconnection including the connections for the x and y switch lines.

A passivation layer 234 in the form of, for example, silicon oxide, isformed on the sensing layer 204 and the indium bumps extend through thepassivation layer into contact with the sensing elements. The tab film210 occupies the space between the passivation layer 234 and a secondpassivation layer 236. Below the second passivation layer 236, thepreprocessors are formed using conventional processes. The tab film 210comprises conductors which connect the preprocessors externally of thedevice.

Each preprocessor has four main devices, a multiplexor, readoutelectronics, a digitizer, and control electronics. The multiplexorselects a pixel or sensing element at a certain (x, y) location to beread out by turning on and off corresponding x and y MOS switches. Theresidual electronic charges released are then quickly collected by thereadout circuit through the indium bump connection. The multiplexor canoperate in either random access mode or scanning mode depending on thecircuit design. The readout electronics may be any one of a number ofconventional readout circuits. However, it is preferred that an analogreadout be obtained and that a direct-injection bipolar current circuitbe used for this purpose. The control electronics provide necessarycontrol and I/0 logic signals to interface to the outside world andcontrol the multiplexor. There may also be a digitizer in eachpreprocessor chip. A flash A/D converter would be adequate for thispurpose. The circuitry of the preprocessor is convention and will not bediscussed in detail here. Essentially this circuitry is presently in usein connection with visible light detector arrays such as the Hitachi MOSimager. The manner in which such circuitry would be used in the presentinvention is therefore obvious to one of ordinary skill in the art.

It is noted that the embodiment of Figs. 12-15 may be in the form of arandom access device or not. Since the cell size is increased, thenumber of cells is decreased per unit area and it is possible to easilyread all of the cells. Consequently, the composition of the preprocessorwould vary depending on whether random access is desired or not.

The embodiment of the invention shown in FIGS. 12-15 solves many of theproblems discussed above in regard to the monolithic detector.Certainly, the sensitivity problem is solved both by the use of anabsorber and by scaling up the size of the capacitor. In addition thepresent embodiment can be scaled up in size to a conventional imageformat size. This can be done with no loss of resolution since, byvirtue if the thin film technology, the external connections whichpreviously gave rise to the "windowpane effect. Finally, by the use ofanalog readout circuitry, a full gray scale can be obtained with asingle exposure.

Another possibility for obtaining good resolution is to use a devicesimilar to the OpticRam and apply an absorber. In fact, it has beenfound that the metallization layer of the IS32 acts as an absorber andthe x-ray detection characteristics of this device could be improved byapplying the metallization layer is such a manner that it overlies allof the cells more uniformly. As is well known, the metallization layeris the layer on an integrated circuit which provides interconnectionbetween the cells. Accordingly, in this case, the metallization layeracts both as an absorber to enhance x-ray detection and as the cellinterconnection layer.

The foregoing description is provided for purposes of illustrating thepresent invention but is not deemed to be limitative thereof. Clearly,numerous additions, substitutions and other changes can be made to theinvention without departing from the scope thereof as set forth in theappended claims.

What is claimed is:
 1. An x-ray imaging system comprising:an x-raysource for producing an x-ray field having sufficient energy such thatcompton scattering and pair production have a higher combinedprobability of producing free electrons in silicon than thephotoelectric effect; and an x-ray detector comprising a solid statedevice having a plurality of layers, one of said layers including asemiconductor material with a plurality of charge storage devices, saiddetector having an extra absorber material for the purpose of enhancingx-ray absorption, said extra absorber material being exposed to saidx-ray field to produce free electrons and being positioned sufficientlyclose to said semiconductor material to permit said free electrons tointeract with said charge storage devices.
 2. An x-ray imaging system asclaimed in claim 1 wherein said detector is placed between said sourceand an object to be viewed such that said x-ray field passes throughsaid detector, impinges on said object and forms an image of said objectin said detector by back scatter from said object.
 3. An x-ray system asclaimed in claim 2 including means for subtracting an image of saidsource from the image produced in said detector.
 4. An x-ray imagingsystem as claimed in claim 1 wherein said detector comprises a thin filmdevice and wherein another of said layers comprises preprocessorcircuitry connected to said charge storage devices.
 5. An x-ray imagingsystem as claimed in claim 1 including a support housing for saiddetector, said support housing having means for removably receiving saiddetector.
 6. An x-ray imaging system as claimed in claim 1 wherein saidcharge storage devices are part of a dynamic random access memory.
 7. Anx-ray imaging system as claimed in claim 1 including means fornormalizing the soak times required for discharging all of the chargestorage devices which are intended to discharge simultaneously.
 8. Anx-ray imaging system as claimed in claim 7 wherein said normalizationmeans comprises means for storing a different normalizing factor foreach of said charge storage devices.
 9. An x-ray imaging system asclaimed in claim 1 wherein each said charge storage device forms asingle pixel of an image formed on said detector.
 10. An x-ray imagingsystem as claimed in claim 1 including an interconnection layer forproviding external connections to said detector, said interconnectionlayer being separate from said layer containing said charge storagedevices.
 11. An x-ray imaging system as claimed in claim 4 including aninterconnection layer for providing external connections to saidpreprocessors, said interconnection layer being between said layercontaining said charge storage devices and said layer containing saidpreprocessors.
 12. An x-ray imaging system as claimed in claim 1including means for reading analog voltages from said charge storagedevices.
 13. An x-ray imaging system as claimed in claim 1 wherein saidextra absorber material is on one side of said layer containing saidcharge storage devices and a metallization layer is on an opposite sideof said layer containing said charge storage devices.
 14. An x-rayimaging system as claimed in claim 1 wherein said detector is anintegrated circuit and the absorber material is a metallization layer ofthe integrated circuit.
 15. An x-ray imaging system as claimed in claim1 wherein each of said charge storage devices is at least about 50microns on a side.
 16. An x-ray imaging system as claimed in claim 1wherein said x-ray detector is about 14 inches by 17 inches.
 17. Anx-ray imaging system as claimed in claim 16 wherein each charge storagedevice is arranged in a basic cell about 0.1 mm square.
 18. An x-rayimaging system as claimed in claim 1 wherein said extra absorbermaterial comprises an increased thickness gate oxide layer.
 19. An x-rayimaging system as claimed in claim 1 wherein said detector is a dynamicrandom access device in which charges are stored on each of said chargestorage devices and dissipated by free electrons resulting from saidx-ray field.
 20. An x-ray imaging system comprising:an x-ray source forproducing an x-ray field having an energy level sufficiently high thatcompton scattering and pair production have a higher combinedprobability of producing free electrons in silicon than thephotoelectric effect; an x-ray detector comprising a solid stateintegrated random access circuit having a semiconductor substrate, aplurality of charge storage devices, and circuit means including aplurality of conductor lines for randomly accessing each of said chargestorage devices, said detector being positioned to be exposed to saidx-ray field such that charges on said charge storage devices areaffected by free electrons produced by said x-ray field; and means forsensing a charge on each of said charge storage devices through saidcircuit means.
 21. An x-ray imaging system as claimed in claim 20wherein said detector is placed between said source and an object to beimaged such that said x-ray field passes through said detector, impingeson said object and forms an image of said object in said detector byback scatter from said object.
 22. An x-ray system as claimed in claim21 including means for subtracting an image of said source from theimage produced in said detector.
 23. An x-ray imaging system as claimedin claim 20 wherein said charge storage devices are divided into groupsto form pixels, each pixel having a plurality of charge storage devicesand means for varying the sensitivity of said charge storage devices ina single pixel to provide a gray scale.
 24. An x-ray imaging system asclaimed in claim 20 including a support housing for said detector, saidsupport housing having means for removably receiving said detector. 25.An x-ray imaging system as claimed in claim 24 wherein said detector hasa thickness on the order of 1/2 mm.
 26. An x-ray imaging system asclaimed in claim 20 wherein each of said charge storage devices iscontained within a cell having a greatest dimension of approximately 10microns.
 27. An x-ray imaging system as claimed in claim 20 wherein saidintegrated circuit comprises a dynamic random access memory circuitincluding at least one sense amplifier for comparing the charge of saidcharge storage devices to a threshold value, and including meansexternal to said integrated circuit for supplying said threshold valueto said integrated circuit.
 28. An x-ray imaging system as claimed inclaim 20 wherein said circuit means comprises transistors for connectingsaid charge storage devices to a voltage source.
 29. An x-ray imagingsystem as claimed in claim 20 including means for moving one of saidx-ray source and said detector relative to the other by a distanceapproximating the distance between charge storage devices in saiddetector.
 30. An x-ray imaging system as claimed in claim 20 includingmeans for normalizing the soak times required for discharging all of thecharge storage devices which are intended to discharge simultaneously.31. An x-ray imaging system as claimed in claim 30 wherein saidnormalization means comprises means for storing a different normalizingfactor for each of said cells.
 32. An x-ray imaging system as claimed inclaim 20 including a plurality of x-ray detectors positioned over oneanother such that the cells of each x-ray detector are staggered withthe cells of the other x-ray detectors such that any one cell of any onex-ray detector is positioned between two cells of another x-raydetector.
 33. An x-ray imaging system as claimed in claim 20 includingmeans for exposing the detector to said source for sequential lengths oftime to produce a plurality of images, and adding said images togetherto provide a gray scale.
 34. An x-ray imaging system as claimed in claim33 wherein the intervals between said sequential exposure times areassigned different gray scale values.
 35. An x-ray imaging system asclaimed in claim 23 wherein said varying means comprises differentthickness oxide layers in said charge storage devices.
 36. An x-rayimaging system as claimed in claim 23 wherein said varying meanscomprises different comparison voltage levels for determining the chargeon said charge storage devices.
 37. An x-ray imaging system as claimedin claim 20 wherein each said charge storage device forms a single pixelof an image formed on said detector.
 38. A method of detecting an x-rayimage comprising:storing charges on a plurality of charge storagedevices formed in a random access integrated circuit; producing an x-rayfield having an energy level sufficiently high that compton scatteringand pair production have a combined probability of producing freeelectrons in silicon greater than the photoelectric effect; exposing anobject to be imaged to said x-ray field; exposing said integratedcircuit to the x-ray field from said object such that said x-ray fieldinteracts with said integrated circuit to produce said free electrons toreduce the charge on said charge storage devices; and sensing the chargeon said charge storage devices.
 39. A method according to claim 38including producing an image having a gray scale by sequentiallyexposing said integrated circuit with different exposure times anddigitally adding the images obtained at each of said exposure times. 40.A method according to claim 38 comprising producing a back scatteredimage by positioning said integrated circuit between said source and anobject to be viewed, causing said x-ray field to pass through saidintegrated circuit and forming an image in said integrated circuitthrough back scattering from said object.
 41. A method according toclaim 38 including normalizing the outputs from said charge storagedevices to compensate for different sensitivities of said charge storagedevices.